Traditionally, the katana is a single-edged sword with a curved blade, known for its exceptional cutting ability. However, there is a rare and less commonly seen variation known as the "ken" or "ninja-to" that features a double-edged blade.

The ken or ninja-to is a straight, double-edged sword that has similarities to the katana in terms of its hilt and overall design. It was associated with certain schools of martial arts and purportedly used by ninja or shinobi. However, it's important to note that the historical accuracy of the ninja-to and its usage by ninja is a topic of debate among historians.

The double-edged katana or ken is distinct from the traditional single-edged katana, which remains the more well-known and prevalent version. The single-edged design of the katana contributes to its cutting efficiency, balance, and overall performance.

It's worth mentioning that the term "katana" is often used generically to refer to a type of Japanese sword, regardless of whether it is strictly a katana or a different variation. In popular culture or fictional works, there may be depictions of double-edged swords labeled as katanas, but from a historical perspective, the traditional katana is a single-edged weapon.